A typical television receiver includes a detector for producing from an input signal an analog, base-band television signal in accordance with the standard used. The analog, base-band television signal includes a horizontal synchronization signal, a vertical synchronization signal and a video portion signal. In a digital television receiver, the base-band television signal is applied to a digital signal processor for producing a video data stream in accordance with, for example, the BT 656 standard. The video data stream includes a corresponding vertical synchronization signal.
When the input signal is absent or too weak, the video data stream will contain invalid data. It may be desirable to detect whether the input signal is absent or too weak. For example, when the input signal is absent or too weak, the detection can be used for blanking the display screen and displaying a banner for the user that indicates absent or too weak input signal.
In carrying out an inventive feature, the video data stream is processed in a microprocessor that performs statistical evaluation of the frequency of the vertical synchronization signal of the video data stream. The statistical evaluation includes computing the statistical mean value and variance of the frequency of the vertical synchronization signal.